1. Field of the Invention
This invention relates to the process and the product by process of the polymers formed by the polymerization of polycyclic ring-opening monomers. More particularly, this invention relates to the novel process and product by process of polymers that are produced with near zero shrinkage or expansion during polymerization and are immediately useful for production of strain-free composites, high strength adhesives, precision castings, binders for propellants and as additives to other monomer mixtures to product a mixture that controls the amount of shrinkage or expansion upon polymerization. The process and product by process of the present invention is also applicable to the production of polymers wherein the prior art practices involved serious factors of binding and shrinkage.
2. Description of the Prior Art
For a number of material applications, such as strain-free composites, high-strength adhesives, precision castings and binders for solid propellants, no satisfactory monomers were utilized in the prior art that will polymerize with near zero shrinkage or even slight expansion. Many composites involving high-strength fibers in a polymeric matrix fail because of either poor adhesion between the matrix and the fibers, because of voids or because of microcracks in the matrix. These problems are related to the fact that when available materials polymerize or cure, a pronounced shrinkage takes place. In bulk plastics, some of these stresses can be relieved only with a total shrinkage in the outer dimensions of the article. Moreover, in a composite, the reinforcing material, which has a high modulus, will often not permit appreciable shrinkage in the overall dimension of the molded object which then causes enormous stresses to be built up in the composite. These stresses are relieved either by an adhesive failure in which the matrix pulls away from the reinforcing fiber or by a cohesive failure in which a void or a microcrack is formed. The complete failure of suitable polymers in the prior art has prevented the formation of strain-free composites, high strength adhesives, precision castings, binders for solid propellants and autopressure casting, heretofore.
The shrinkage that occurs during polymerization has been assertained by the applicant to arise from the fact that in the monomer starting materials, utilized in the prior art for polymer production, the monomers are located at a Van der Waals' distance from one another while in the corresponding polymer, the monomeric units move to within a covalent distance of each other. Simply, the atoms are much closer to one another in the polymer than they were in the original monomer. Nichols, et al., [Ind. Eng. Chem., 42, 292 (1950)] has calculated the shrinkages for addition polymerizations and has found shrinkage ratios of from 34% for vinyl chloride to 6% for 1-vinylpyrene. The general trend found was the larger the monomer the smaller the shrinkage; that is, when the double bond is a relatively small portion of the molecule, the percent of shrinkage is much less. For this reason, the use of prepolymers were needed to reduce the actual shrinkages that occured during the final polymerization. The principle use of these addition polymers are in the form of thermoplastic molding powders wherein the shrinkage during polymerization is of little importance; and not in casting, wherein the shrinkage is a serious problem.
More drastically, in a condensation polymerization, the shrinkage is apt to be greater than that which occurs in many addition polymerizations since a small molecule is eliminated during the formation of the new bond. Thus, in addition to the shrinkage that takes place when the molecules make the transition from a Van der Waals' distance to a covalent distance, an additional shrinkage occurs because the volume occupied by the atoms of the small molecule is now vacant, resulting in a still further shrinkage. In phenolic resins where water is split out, shrinkage is such a termendous problem that very little molded material is made without the use of fillers, which give the final articles some dimensional stability, but produces enormous stresses to be built up in the composite, and which are only relieved by adhesive failure or by cohesive failure in which a void or microcrack is formed. De Boer [Trans. Faraday Soc., 32, 10 (1936)] has calculated that if all the primary valence bonds are formed in a heat-hardened phenolic resin, the theoretical strength should be 4000 Kg/mm.sup.2. Actually the usual strength of phenolic resins is about 8 Kg/mm.sup.2, 1/500 of the expected strength. Applicant's invention seeks to remedy this long sought need.
In ring-opening polymerization, the shrinkage may be less since not only is a small molecule not eliminated during the polymerization reaction, but for every bond that goes from a Van der Waals' distance to a covalent distance another bond goes from a covalent distance to a near Van der Waals' distance.
The shrinkage ratios during ring-opening polymerization run from 23% for Ethylene oxide to 3% for 5-Oxa-1,2-dithiacycloheptane and 2% for the dimethylsilane oxide cyclic tetramer; with the general rule that as the ring that is opened gets larger, the shrinkage gets smaller; or the closer the monomer resembles the final polymer, the smaller the shrinkage. The prior art has shown no examples of polymerizations with essentially zero shrinkage or which expand during polymerization.
The only previous use of a polycyclic ring-opening compound in the polymer area is the use of spiro ortho esters as chain-degradation stoppers to prevent the decomposition of oxymethylene polymers (Japan '65 3708, Japan '69 28,111, Japan '67, 3496, Fr. 1,409,957). Normally oxymethylene when heated will revert back to formaldehyde by unzipping. The thrust of the Japaness patent was that by placing a group into the backbone which will change the mechanism of decomposition it will therefore prevent the decomposition from occuring. The spiro ortho ester was not used for its volume properties and, moreover, the claim was limited in its scope to a mixture containing less than 15% spiro ortho ester; the strongest proof that the volume properties of these compounds were unrecognizable in the art. The prior art has shown no examples of polymerizations with essentially zero shrinkage or which expand during polymerizations even though a long sought need has existed for precision casting materials and strain-free composite binders.